Warming world: Satellites and weather balloons

Warming world: Satellites and weather balloons

Image: NASA

Image: British Antarctic Survey

Image: NASA

Image: NOAA

Image: Bredk / creativecommons.org

Image: Q. Fu / IPCC

Scientists record temperatures in the troposphere and stratosphere using both satellite and radiosondes. Radiosondes measure air temperature using thermometers carried aloft by balloons. Satellites measure the energy given off by the Earth’s atmosphere, from which scientists calculate the temperature. Both radiosonde and satellite measurements show a long-term warming trend in recent decades.

Earth-observing satellites have been orbiting our planet since the late 1970s. Some carry instruments that measure the microwave energy given off by the atmosphere, from which scientists can estimate temperatures in different atmospheric layers. The initial analysis of the satellite data showed no significant warming trend in the troposphere, contradicting both surface measurements and radiosondes. But these initial calculations didn’t correctly account for the fact that satellite orbits had gradually drifted, leading to measurements being taken slightly later each day. Once these errors were recognised and corrected, satellite data showed a warming of between 0.1 and 0.2 °C per decade. This is in broad agreement with surface and radiosonde measurements.

Filled with hydrogen or helium, weather balloons carrying scientific instruments called radiosondes soar up to 20 km into the atmosphere – higher than most aeroplanes fly. The radiosonde, which usually weighs less than half a kilogram, measures temperature as it rises through the atmosphere, beaming its data back to laboratories. The radiosondes eventually fall back to Earth, but are rarely recovered. Scientists record their data and combine the individual measurements into a global average temperature. Beginning in 1958, radiosonde data show a warming trend of between 0.1 and 0.2 °C per decade. This is in broad agreement with satellite and surface measurements.

All objects give off energy – the Sun, the Earth, even the human body. The intensity and frequency of the energy depends on the object’s temperature: the hotter the object, the higher the energy’s average frequency and intensity. Because temperature varies throughout the Earth’s atmosphere, different areas give off energy at different frequencies and intensities. So by measuring the energy given off by different atmospheric layers, scientists can estimate their temperatures. Satellites measure the intensity of microwave energy from the lower troposphere, the mid-to-upper troposphere and the lower stratosphere. These measurements began in 1978, enabling scientists to estimate variations in temperature in these layers over the past 30 years.

Climate models simulating the Earth’s atmosphere predict that when the world warms up, the upper troposphere should warm faster than the surface, especially in the tropics. But initial weather balloon and satellite measurements suggested that the upper troposphere was hardly warming at all. Some measurements even showed a slight cooling trend. Some scientists thought there might be a problem within the climate models, causing them to simulate the atmosphere incorrectly. But later measurements taken over longer time periods show a warming trend in the upper troposphere after all. There are still many uncertainties in the measurements, but the most recent analyses are in broad agreement with climate model predictions.

The layers of the atmosphere, with the Space Shuttle Endeavour in the foreground.

In contrast to the warming trends observed by satellites and weather balloons in the troposphere, both types of measurement have recorded a marked cooling trend in the stratosphere. Though this might seem strange, a cooling stratosphere is actually consistent with what scientists expect from global warming caused by increased greenhouse gas levels. Stratospheric temperature depends partly on the amount of energy it receives from the troposphere below it. Human activities are emitting greenhouse gases, trapping more heat energy in the lower atmosphere, causing the stratosphere to cool down while the troposphere warms up. Ozone depletion, caused by human emissions of chlorofluorocarbon (CFC) gases has also played a part.

The world’s oceans are a vast reservoir of energy. Because water warms more slowly than air, the rise in sea surface temperature is slightly slower than the corresponding rise in surface air temperature. But there has been a marked increase in the amount of heat energy stored in the oceans. Temperature probes take measurements from the surface of the sea to depths of several hundred metres below. Scientists have compared probe measurements with satellite data to calculate changes in the heat energy contained in the ocean’s surface layer. The calculations indicate that more than 80% of the increased heat energy in the climate has been taken in by the oceans.

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There are many institutions and organisations around the world researching climate science, how our climate is changing, and ways of responding. Here are just a few…

British Antarctic Survey (BAS)

Department for the Environment, Food and Rural Affairs (Defra)

Department of Energy and Climate Change (DECC)

Energy Saving Trust (EST)

Environmental Change Institute (ECI)

European Space Agency (ESA)

The Geological Society (GS)

Grantham Institute for Climate Change (GICC)

Intergovernmental Panel on Climate Change (IPCC)

Met Office (MO)

National Academy of Sciences (NAS)

National Aeronautics and Space Administration (NASA)

National Oceanic and Atmospheric Administration (NOAA)

National Oceanography Centre (NOC)

The Royal Society (RS)

Tyndall Centre for Climate Change Research (TCCCR)

UK Climate Impacts Programme (UKCIP)

United Nations Framework Convention on Climate Change (UNFCCC)

World Climate Research Programme (WCRP)

World Meteorological Organization (WMO)

Satellite

Any object in orbit around a larger body. Satellites can be natural – such as the Moon – but more often the term is used to refer to an object made by humans which is orbiting the Earth. Scientists use these satellites to collect data about the Earth, its weather and its climate.

Radiosondes

Sensors attached to a weather balloon that measure atmospheric conditions. As the balloon rises upwards, the radiosonde takes various measurements including temperature, pressure and humidity and sends the results back to scientists on the ground.

Microwave energy

A very-low-frequency wave of energy.

Atmospheric layers

Scientists find it useful to study the atmosphere by dividing it into different layers with different properties. The bottom layer, up to a height of roughly 10 km, is known as the troposphere. Above this, between 10 and 50 km, is the stratosphere.

Weather balloon

A balloon used to measure conditions in the atmosphere. Scientists attach a measuring device called a radiosonde to the balloon before sending it floating upwards to take measurements and transmit them back to scientists on the ground.

Scientists around the world release about 1600 weather balloons each day.

Frequency

A property of waves equal to the number of waves per second. Visible light, ultraviolet, infrared and microwave energy are all waves of energy moving through space. The difference between them is that they have different frequencies.

All objects with a temperature above absolute zero emit waves of energy with frequencies that depend on the object’s temperature. High-frequency energy such as ultraviolet energy is emitted from very hot objects, for example the Sun. Lower-frequency infrared energy is emitted from warm objects such as the Earth.

Climate model

A mathematical simulation of the climate that recreates the interactions of the atmosphere, ocean, land and their ecosystems.

Scientists use models to explore how the climate system works and project how it might change in the future.

Models can vary from simple calculations to complex computer programs, but all are based on the laws of physics and the known properties of the different components of the climate (such as clouds, temperature, rainfall). They produce a range of possible future outcomes partly depending on the assumptions made about how much human activities will influence the climate over coming decades and partly because some parts of the climate system are still not fully understood.

Global warming

An increase in the global average temperature at the Earth’s surface. Global warming can have both natural and human causes.

Global warming is commonly used to refer to the warming effect of increased amounts of greenhouse gases emitted by human activities into the atmosphere.

Ozone depletion

A steady decline in the amount of ozone in the stratosphere, measured since the late 1970s. As well as a steady global decline year on year, a large seasonal decline occurs over the Earth’s polar regions.

This is known as the ozone ‘hole’. Scientists showed that the ozone in the atmosphere reacts with man-made chemicals, most of which have since been banned.

The ozone hole does affect the climate around the poles, but is not directly part of the process of human-induced global warming.

chloroflurocarbons

There is no definition for this glossary item.

Temperature probe

An instrument used to measure the internal temperature of a particular material or substance. Scientists use temperature probes to measure temperature inside glaciers and below the ocean’s surface.

Climate

A summary of the weather in a particular region over a period of at least ten years, but more commonly defined over 20 - 30 years. The climate describes both the average weather conditions (for example temperature, rain, snow and wind) in a particular region as well as the extremes.